GERMANY. 479 



men in the United States who are capable of properly selecting and preparing the 

 wood. 



Thus prepared, the sticks are brought to the factory and passed one by one 

 through a machine armed with a powerful revolving knife, which at each revolu- 

 tion slices obliquely through the log, cutting off a chip about 3 inches in length, 

 which is split by impact of the knife into shreds varying from mere splinters to 2 or 

 3 inches in thickness. From the cutting machine the chips are carried to the boilers, 

 which in the Mitscherlich process are made large enough to contain not less than 

 10 tons of wood. Such a boiler is from 30 to 40 feet in length and 12 to 14 feet in 

 diameter, and is set either vertically or horizontally in strong masonry. The boiler 

 is pierced with manholes for introducing and removing the wood, and is rigged 

 with a system of branching pipes, controlled by valves and leading to the steam 

 generator, the water reservoir, and the tanks where the chemical solution is stored 

 after preparation by one of two processes which will be hereinafter described. 

 Gauges and cocks are also provided to enable the attendant to read and regulate 

 the pressure in the boiler and to draw off samples of the liquid contents, from which 

 the progress of the operation can be accurately observed. 



The treatment of the wood in the boiler includes two stages or operations digest- 

 ing and boiling the first being performed with live steam and spent bisulphite 

 solution, the second with fresh bisulphite and steam heat applied through coils of 

 hardened lead pipe laid round the lower portion of the interior to about one-third 

 the height of the boiler, the proportion being about 50 square feet of heating surface 

 to each 1,000 cubic feet of its capacity. As sulphurous acids attack iron and steel 

 with great avidity, it was found necessary to make these steam coils of lead and to 

 protect the interior of the boiler with a lining of acid-resisting material. For the 

 latter purpose lead was likewise at first used; but Dr. Mitscherlich made an impor- 

 tant improvement in the substitution of hard, glazed earthernware tiles or bricks, 

 firmly laid against a backing of sheet lead closely packed between the tiles and 

 the steel shell of the boiler. These lining bricks are made with edges tongued and 

 grooved, and the lining, when properly made, lasts a long time and thoroughly 

 protects the shell, the point of greatest danger being the edges of the upper man- 

 holes, which are exposed to the action of sulphurous gases, rather than liquid 

 solutions. 



The boiler having been filled with wood and the manholes securely closed, steam 

 is turned in from the generator, and at the same time there is introduced through 

 a pipe at the bottom waste or spent solution of bisulphite of lime. As the pressure 

 increases the steam penetrates the pores of the wood, expelling the air and opening 

 the way for the chemical solution, which gradually rises, submerging and penetrat- 

 ing the wood and replacing the steam, attacking in proportion to the strength of 

 the lye the resinous matter in which the fiber is imbedded. This steaming and 

 softening process, technically called "digesting," occupies from eight to ten hours. 

 When it is finished the steam is shut off, the lye withdrawn, and the fresh solution 

 of strong bisulphite of lime turned in. This cold injection condenses the steam, 

 producing a vacuum, which sucks the solution upward without the aid of pumping 

 until the requisite quantity has been injected. The valve is then closed and steam 

 turned into the coils, and the second operation "boiling" begins. This com- 

 mences gradually as the temperature rises and continues from eighteen to twenty 

 hours. This gradual raising of the temperature by the heat of the steam coils is a 

 very important part of the process and is conducted with great care. At about 158 F. 

 the chemical action of the bisulphite upon the resinous incrusting matter begins. 

 At 212 steam begins to generate, and the pressure forces the liquid sulphite into 

 the innermost pores of the wood. If the heating has been properly regulated, the 

 pressure at the end of thirty-six hours will have risen to from 45 to 50 pounds to 



